1. Field of the Invention
This invention relates to diaphragm driving mechanisms, and more particularly to an electromagnetically operated diaphragm driving mechanism.
2. Description of the Prior Art
FIGS. 1 and 2 illustrate a conventional example of the diaphragm mechanism with its drive source located externally. 1 is a body tube of a lens mounting fixedly secured to a camera housing (not shown). Fixedly fitted in the body tube 1 is an apertured disc 2 having a plurality of holes 2a in a common circle with its center at the optical axis in equally spaced relation. Another apertured disc 3 as a diaphragm drive member has a plurality of radially elongated slots 3a in equally spaced relation and is positioned at a distance from the first disc 2 to be rotatable about the optical axis. Positioned between these two discs 2 and 3 are a plurality of diaphragm blades 4 out of contact with both discs 2 and 3 by a gap "a". Each of the blades 4 has pins 4a and 4b on opposite surface thereof which extend respectively into the hole 2a and the slot 3a. A retainer ring 5 behind the diaphragm drive disc 3 is fixedly mounted to the body tube 1 by fastener screws 6. A drive connection rod 3b extends rearwardly of the diaphragm drive disc 3 through an arcuate opening 5a of the retainer ring 5 into a control mechanism for the diaphragm in the camera body.
In operating the conventional example of the diaphragm of such construction, the drive connection rod 3b is turned about the optical axis, so that the diaphragm drive disc 3 turns relative to the disc 2, whereby the draphragm blades 4 are driven to pivotally move.
By the way, in order to ensure that the diaphragm blades 4 operate smoothly, it has been the general practice to make provision for the spacing "a". For high performance of the optical instrument, the diaphragm blades 4 must be positioned to a high accuracy in the axial direction. Despite this, use of the gap "a" has been avoided with the result that the diaphragm blades 4 constantly fluctate in the axial direction. This leads to deterioration in the optical performance. In such a manner, the requirement for smooth operation is not compatible with that for high accuracy, causing a very serious problem.
Further, recently, a drive source using electromagnetic force had been increasingly used in operating the diaphragm. FIG. 3 illustrates a conventional example of such electromagnetically driven diaphragm mechanism. Because this type of motor or electromagnet as the drive source cannot exert a force of larger magnitude than its size limits, it is necessary to provide a gap similar to that in the first conventional example described above.
In FIG. 3, a body tube 11 of a lens mounting contains stationary and rotatable apertured discs 12 and 13 between which diaphragm blades 14 are positioned at a distance "b" from each of the discs 12 and 13. The diaphragm drive disc 13 is connected to a magnet ring 15. A retainer ring 16 behind the magnet ring 15 at a gap "b'" is fixedly mounted to the body tube 11 by screw fasteners 17. On the outer periphery of the magnet ring 15 there is a driving coil 18 which is fixedly fitted in the inner diameter of the body tube 11, and is spaced by a short radial distance from the magnet ring 15.
In operating the electromagnetic drive type diaphragm mechanism of such construction, when the driving coil 18 is supplied with current from a drive control circuit (not shown), the diaphragm drive disc 13 turns moving the diaphragm blades 14.
However, even in this conventional example of the electromagnetic drive type, because of its having the spaces "b" and "b'", the diaphragm blades 14 constantly change their axial positions, causing a large problem by deteriorating the optical performance.
Also, as shown in FIGS. 4A and 4B, the angular speed at which the diaphragm drive disc 13 moves varies with time (FIG. 4A), and the angle of rotation varies with time (FIG. 4B). Because the diaphragm drive disc 13 and the diaphragm blades 14 have inertia, if the current supply to the driving coil 18 is cut off at a time, t.sub.0, when the angle of rotation reaches a target value .theta..sub.0, the diaphragm drive disc 13 continues to rotate until a time point t', and, therefore, overruns by an angle .DELTA..theta.. For this reason, it is impossible to set the aperture size in a correct value.
As the means for eliminating the above-described problem mention may be made of means for electromagnetically braking the disc, and high degree feed-back control means. The former has the problem of increasing space requirements. The latter necessitates a high degree of technology. Both involve factors which increase costs and which cause difficult manufacturing problems.